Part 5 - Radar Systems Flashcards
What does radar stand for?
Radio Detection and Ranging
What are the two basic classifications of radar?
Primary and secondary
An active radar does what?
It transmits a high powered RF pulse and receives a weaker return that has reflected off a target
Stimulate and response describes what type of radar?
Secondary radar
Pulse modulation is a process normally used on aircraft communication systems, true or false?
False, it’s used on radar
A transponder which is part of a secondary radar system will give a “reply” once it has been “interrogated” by a primary radar - true or false?
True
What frequencies are normally used for airborne radar?
1 GHz to 15 GHz
A radar transmits short bursts of EM energy to receive what information about a target?
Range, bearing and height
Inside the transmitter, what is the purpose of the modulator?
Functions as an electronic high power switch, triggered by pulses from the master timing unit
Will produce pulses of the correct pulse duration and frequency to switch the magnetron on and off
The master timing unit output pulse is what type of pulse?
A synchronisation pulse which recur at precise and regular time intervals
Determines the pulse recurrence frequency (PRF) of the equipment
If the Pulse Repetition Period (PRP) is 2000μs, and the pulse duration is 1μs, what will be the resting time?
1999μs
The number of radar pulses produced in 1 second is known as?
Pulse Repetition Frequency (PRF)
If a radar has a PRF of 500 Hz, what will the PRP be?
2 ms
The ratio of the pulse duration to the PRP is known as what?
The pulse duty factor or duty cycle
If the pulse duration is 1μs and the PRP is 3000μs, what is the pulse duty cycle?
1/3000
What is the formula to calculate the mean power (PA) of a radar pulse?
PMTDPRF
If a pulse has an average power = 240W, max power = 400 kW and PD = 1μs, what is the PRF?
Mean power/(maximum power*PRF)
Parabolic and flat plate are type of what?
Radar aerial
A TxRx switch does what?
Automatically connects the Tx to the aerial for the duration of each transmitted pulse, then switches so that the Rx is connected to the aerial during the interpulse period
Also known as a Duplexer
A Rx must be very … in order to detect the weak radar returns from distant targets?
Sensitive
How long will a radar pulse take to travel 1nm and back?
12.36μs
Calculate the distance of an object that provides a return pulse after 123.6μs?
10nm
On a partial PPI radar display, how is the bearing read?
From the dead ahead position, to left and right (port and stbd)
A radar pulse travels 100nm with a transmission power of 1MW. If the power is doubled, what will the range be?
Doubling the power, multiplies the range by 1.19, therefore 119nm
In a radar’s pulse, there needs to be a minimum of 200 cycles, true or false?
True
What does the PRF of a radar depend on?
- Max range required
- Scanning speed
- Mean power
- Required target definition
A target that is outside of the range of a radar is displayed on the indicator at a closer range than it actually is - what is this known as?
Second trace returns
If the scanning speed of a radar is increased, a higher PRF is required - true or false?
If the aerial scanning speed is high, the PRF must also be high, otherwise targets may be missed because the aerial will have turned through part of its ‘scanning’ angle during the pulse interval
If the pulse duration and listening time of a radar pulse is different, is the wave symmetrical or asymmetrical?
Asymmetrical
What is decay time?
Time taken for a pulse to fall from 90% of its amplitude to 10%
What is pulse duration?
Pulse duration measured from the 50% of the maximum value of the pulse
How is the mean value of a rectangular waveform calculated?
Max amplitude divided by the number of periods in one cycle of the waveform (pulse taken as 1 part, and number of parts in the rest period equivalent to the same length of time as the pulse)
What classification of radar is a weather radar?
Primary
What is a bearing in “azimuth”?
A bearing to the left and right of the centre line, in the horizontal plane
How is the bearing measurement achieved in a weather radar?
Making sure the timebase line is in the same direction as the weather radar scanner
Why is the weather radar beam in azimuth narrow?
If the beam was too wide, then returns from several targets ahead would tend to merge together and be less distinct
Narrow beam will improve target resolution
How are the signal strengths indicated on a weather radar display?
Signal strength within the returns are identified by larger returns creating greater bright up and therefore a more intense display, with the most intense displays depicting the most dangerous cells
How does the digital weather radar display differ from the traditional weather radar display?
Digital weather radar returns are stored in a memory and the display is updated from this memory rather than directly from ‘real-time’ returning energy
What colour is the most severe weather displayed on a weather radar system?
Magenta or red
What effect does range selection have on the weather radar rest time?
No effect, it only changes displayed range and not transmitter/receiver range
What can be used to estimate the distance of a radar target or weather front on the display?
Semi-circle range markers
Which mode is used when a smaller sweep angle of the radar is selected?
Sector scan mode
Antenna tilt allows the weather radar to have what capability?
Tilting up to eliminate ground returns or to estimate approximate heights of targets ahead
Tilting down to give a deliberate ‘look down’ capability, which creates the necessary ground returns used in the mapping mode of operation
How is antenna stabilisation achieved in a weather radar?
Error signals are fed from a reference source, such as INS/VG, and these can be corrected by the weather radar gimbals
What ensures that the brightness of a target displayed will be independent of range?
Sensitivity time control (STC)
Why is sensitivity time control necessary?
To ensure that close targets don’t swamp the display, even though they may be harmless
Any attempt to reduce gain for nearby harmless targets, may result in distant dangerous targets not being amplified enough to bright up the screen
What is the purpose of AGC in weather radar?
Reduces the receiver gain when the noise level reaches a predetermined level to help remove noise from the display
What effect does a smooth sea have on the doppler radar?
Typically, more echo energy is reflected at the rear edge of a forward beam than at the front edge, but this is masked by surface irregularities
Smooth sea doesn’t mask the variations in energy levels, which moves the mid frequency point, giving an incorrect groundspeed
What shape of beam will a traditional radar use for mapping mode?
Cosecant shaped beam
What are the advantages of the flat-plate scanner over a parabolic?
The flat plate scanner is more efficient and produces waveforms with less sidelobes
The Bendix system, which is a parabolic scanner with a modified reflector, can create what type of radar transmission beams?
Pencil and cosecant
What properties must a radome have?
- Must pass at least 90% of transmitted energy
- Must possess high structural strength
- Must resist spark discharges and lightning strikes
A double skinned radome must have the two skins separated by what distance?
1/4 wavelength to ensure that any reflected energy from both skins will cancel out
What is the principle of operation of doppler turbulence detection?
Detects statistical variance in droplet velocity, with the broader the spectrum of velocities the greater the turbulence
What are the two outputs of the doppler system?
Ground speed and drift angle
How is velocity measured in a doppler system?
Relative velocity between the transmitter and the target which gives a difference frequency
What is the formula used to calculate the difference frequency?
Difference frequency is given by (2Vrftx)/c
Where Vr is component of velocity along sighting-line of beam
What will the measurement of doppler shift allow?
A direct measurement of a velocity or speed
What are the frequency bands used for airborne doppler systems?
8.8 GHz and 13.3 GHz
In a two beam doppler system, where the beams are radiated from the front left and front right of the aircraft, what will a greater doppler shift on the right beam indicate?
The aircraft is drifting to the right
What is the general beam arrangement of a Janus doppler system?
Two forward beams and one backward beam
What are the three types of doppler transmission systems?
Pulsed, CW and FMCW
How can the cross coupling effect be reduced in a pure CW doppler system?
Using two aerials, one for Tx and one for Rx, shielded from each other by the insertion of a metal baffle plate between the two arrays
Which type of doppler transmission is largely immune to the effects of cross coupling?
FMCW
Which type of doppler transmission suffers from the effect of “height holes”?
FMCW
In doppler, what is the error known that refers to a change in beam reflections due to the sea surface, and how can it be reduced?
Sea bias
Use of a Land-Sea switch
What does the Low Range Radio Altimeter measure?
The vertical distance from the aircraft to the terrain below
What is the normal maximum operating height of the LRRA?
2500 feet
What frequencies are used by radio altimeters?
C band - 4.3 GHz
What is a decision height warning?
Visual and aural warning of passing below a crew selected height
On initial touchdown, what height will the RadAlt display?
0 feet
What is the principle of operation of a RadAlt?
RadAlt transmits frequencies approximately 50MHz above and below centre frequency
Frequency F1 is transmitted, reflects off ground and returns to the Rx
This frequency is compared to current transmitted frequency from Tx, call this F2
Difference between frequencies gives an indication of elapsed time and therefore distance to the reflective surface
What are second echoes in RadAlt and what height will it produce?
Transmitted signals from RadAlt which have carried out two return trips due to a second reflection from the aircraft underside
Double the height
What is the calculation required to overcome Aircraft Installation Delay in a RadAlt system?
Rx antenna distance to the ground + Tx antenna distance to the ground + (co-ax length/0.67)
To whom will the basic aircraft transponder communicate with?
Only to a ground station
What information can the basic transponder transmit to ATC?
- Echo of the aircraft position
- Identification of the aircraft which produced the echo
- Distance and bearing to the aircraft
- Aircraft height
- Certain emergency situations
What frequencies will the transponder system use?
Receive - 1030 MHz
Transmit - 1090 MHz
The same is true but in reverse for the ground station
Why are the transmit and receive frequencies different for IFF?
To stop cross over and swamping of their own receivers by echos
In the standard transponder system, what is the pulse duration of the P1, P2 and P3 pulse?
0.8 microseconds
What are the pulse durations for the different modes of the basic transponder?
Mode A - 8 microseconds
Mode B - 17 microseconds
Mode C - 21 microseconds
Mode D - 25 microseconds
When the aircraft transponder replies to ATC, what is the construction of the message?
Framing pulses F1 and F2 are spaced 20.3 microseconds apart
How wide are the pulses in a basic aircraft transponder transmission?
0.45 microseconds
What is the result when the pilot is told to “squawk ident”?
Pilot presses the IDENT button, pulse added 4.35 microseconds after F2 called the identification pulse
Up to 20 seconds, causes the aircraft echo on the display to flash or otherwise be highlighted
A “mode A” code of 6537 will select which lines in the encoder?
A4A2 (6) B4B1 (5) C2C1 (3) D4D1 (7)
What does the radio code 7600 indicate?
Radio failure
How is side lobe suppression achieved in the traditional transponder system?
Control pulse, P2, is transmitted 2 microseconds after P1 on an omnidirectional antenna to a range which will completely overlap the sidelobes of the main beam
If the control pulse is within 9 dB of the amplitude of the main beam, the aircraft is within a sidelobe and the interrogation can be ignored
What is the principle of operation of the decoder for interrogations from ATC?
Video output of the detector is fed to a delay line and a coincidence gate concurrently
Delay line will ensure that P1 of the delay line meets P3 at the coincidence gate, as long as the mode switch on the controller is on
Output pulse will be fed to the encoder
What is ‘fruiting’ with regards to PPI indications?
Unwanted replies to interrogations appearing on an ATC display
What are some of the causes of ‘fruiting’?
- Replies from an aircraft, which had been interrogated by other ground receivers
- Aircraft replies to side lobe interrogations
What two standard techniques are used to give ‘de-fruiting’?
- All ground interrogations work at slightly different PRF, so ground receivers will only process their own replies
- Side lobe suppression
What is Group Count Down?
Aircraft has a maximum duty cycle to prevent overloading, affected by the number of interrogations and the mode and code used for the reply
If the allowable duty cycle is likely to be exceeded, internal circuitry in the transponder de-sensitises the receiver
The transmitter therefore will only reply to the nearest interrogator and not the distant ones
What do the higher levels of automation within Mode S transponders include?
- Bi-directional air-to-ground data downlink
- Extended length messages
- Multisite message protocol
How will a mode S transponder reply to an all call interrogation?
Mode S format that includes the 24 bit mode S address
What are the two types of digital modulation techniques that a mode S transponder responds to?
- Pulse amplitude modulation PAM
- Differential phase shift keying DPSK
Which pulse stops a mode S transponder replying to an ATCRBS ground station?
P4
What does the mode S DPSK interrogation consist of?
- P1-P2 pulsed pair followed by a long pulse P6
- 56 or 112 bits within P6
How is coordination between two TCAS equipped aircraft managed?
The mode S transponder acts as the communications link to co-ordinate avoidance procedures
Onboard coordination between TCAS and mode S transponder is conducted via the TCAS co-ordination high speed ARINC 429 databus
What is a resolution advisory?
An aural or visual display instructing the pilot on actions to be taken or avoided in order to maintain or increase vertical separation relative to an intruding aircraft
During which advisory, should the aircrew try and make ‘visual contact’ with the intruder?
During TA
What is the caution area?
An area of airspace which begins 35 to 40 seconds from the time an intruder aircraft is predicted to enter the TCAS aircraft’s collision area
What is an intruder?
An aircraft equipped with an operating Mode A, Mode C or Mode S transponder that has satisfied the TCAS II intruder detection logic
Predicted to enter the TCAS aircraft’s collision area
What will a Mode C reply give?
Aircraft altitude in addition to Mode A information when interrogated
What is ‘other traffic’?
Any transponder replying traffic not classified as an intruder or proximate traffic, within 2700 ft vertically and the range of the display
What is ‘proximity traffic’?
Any transponder replying traffic within a 6 nautical mile radius and within 1200 feet vertically of the TCAS aircraft
When will RAs occur?
Designed to occur when an intruder’s predicted flight path is within 20 to 30 seconds from entering the TCAS aircraft’s collision area
What is a corrective advisory?
A resolution advisory that instructs the pilot to deviate from the current vertical rate
What is a preventative advisory?
A resolution advisory that instructs the pilot to avoid certain deviations from current vertical rate
What are the priority of the environmental alerts?
- Stall
- Windshear
- GPWS
- TCAS
When will TCAS revert to Traffic Alert Only Mode and what will be inhibited?
Inhibited by stall, windshear or GPWS
Aural alerts are inhibited
In the simplest terms, how does a TCAS work?
TCAS II continually surveys the airspace around an aircraft, seeking replies from other aircraft in the vicinity via their transponders
Transponder replies are tracked by the TCAS system and flight paths predicted based upon these tracks
What will the TA show about the intruder?
- Range
- Bearing
- Altitude of the intruder aircraft relative the TCAS aircraft
What will the TCAS announce when the intruder aircraft no longer conflicts with the collision area of the TCAS aircraft?
‘Clear of conflict’
What type of alerts will be generated against intruder aircraft whose transponders only reply in Mode A?
TAs
How will Mode S transponders announce their presence with regards to TCAS?
Transmitting a squitter message, once each second
What does TCAS do after Mode C or Mode S intruders are detected?
TCAS places the intruder in TRACK
Up to 30 intruders can be tracked
What is TAU?
Maximum time the flight crew need to discern a collision threat and take evasive action
How is TAU calculated?
Speed and path of host (in nautical miles) / Closure rate of intruder (in knots)
Why is time-to-go to the closest point of approach (CPA) more important than distance-to-go to the CPA?
The system is designed in this way to ensure the pilot will always have the same amount of time to react, regardless of closing speed
What is airspace density and what volume can TCAS hold?
Number of aircraft per square nautical mile
0.3 transponder equipped aircraft per square nautical mile
What symbols are associated with TCAS?
Hollow cyan diamond - other transponder replying traffic not classified as an intruder or proximate traffic, within 2700 ft vertically
Solid cyan diamond - aircraft within 6 nm, within 1200 ft vertically
Solid amber circle - intruder aircraft entering the caution area 35-45 seconds from the TCAS II collision area
Solid red square - intruder aircraft entering the warning area 20-30 seconds from the TCAS II collision area
What is ‘Whisper Shout’?
A sequence of interrogations that are transmitted at different power levels
The airspace around the aircraft are already divided into space in azimuth, but whisper shout allows the area to be divided by range
